The invention relates to an electrochromic device of the type comprising at least one substrate and a structure of at least partly superimposed layers, where the structure comprises at least one layer of electrochromic material and a layer of electronic insulating transparent ion-conducting solid electrolytic material, as well as to uses of such electrochromic devices.
Devices of this type allowing the passage of electromagnetic radiation, particularly the passage of visible light, to be regulated by varying an electric potential difference are known. This variation of the potential difference causes the layer of electrochromic material to vary the transmittance thereof.
One possible use for these devices is in rear-view mirrors for motor vehicles. Under certain circumstances, for example when driving at night, the light reflected by the rear-view mirrors can dazzle the driver. Therefore, various devices have been designed which, based on the electrochromic effect that certain materials have, allow the transmittance of one or more of the layers forming the mirror to be regulated, so as thereby to reduce the amount of reflected light and avoid dazzling.
Various factors are involved in the design of an electrochromic device and, in certain cases, they have opposite effects. A conventional electrochromic device, for example a rear-view mirror, usually comprises a glass substrate on which there are deposited a layer forming a transparent electrode, a layer of electrochromic material, a layer of electrolytic material, a layer of another electrochromic material complementary to the first layer (i.e., which reacts to the electric polarity in the opposite direction to the first layer, in such a way that both layers vary their transmittance in the same direction, usually also called a contraelectrochromic material) and a layer forming a reflective metallic electrode. A second glass substrate usually seals the ensemble. Both the materials and the thicknesses of the layers have a great influence on parameters such as the transmittance of the ensemble, the transmittance variation of the ensemble, the response rate of the ensemble to the application of a particular voltage, etc. Obviously, it is of general interest to have a high maximum transmittance, a lowest possible minimum transmittance, and the fastest possible response rate. In the particular case of the use of electrochromic devices to rear-view mirrors, all these properties are extremely important. Dazzling is caused by a very great difference of light intensity, whereby the transmittance of the electrochromic layers of the rear-view mirror must be greatly varied to counteract this light intensity difference and, also, it must be done at high speed, since otherwise dazzling has already taken place. Nevertheless, all the layers of the device must have a high transmittance so that the mirror will not have a dark appearance during daylight driving.
It is an object of the invention to overcome these drawbacks. This aim is achieved by an electrochromic device of the type first mentioned above, wherein at least one of said layers is nanostructured, i.e., has a nanostructure.
In general, a nanostructured material is a material which: a) has a structure with a crystalline order of nanometric range (with domains in the range of 2-20 nm); or, b) having crystalline domains of nanometric dimensions (1-20 nm) embedded in an amorphous matrix, of the same or a different compound; or, c) having a structure formed by multiple layers of nanometric thickness (2-100 nm), alternating between two different materials or alternate layers of the same material but with a different structure (for example, combination of amorphous and nanocrystalline), different degree of oxidation and/or different stoichiometry.
This nanostructure has a great influence on various physical properties of the material. Thus, a nanostructured layer has improved conductivity properties relative to a conventional layer. Likewise, a nanostructured layer of electrochromic material has a greater ion storage capacity, as a result of the increase in the number of interfaces and of the structural disorder. The physical and chemical properties related to the new structure (ion transportxe2x80x94entrainment and diffusionxe2x80x94, optical absorption, device switching speed) may be optimized, in accordance with the design of the layer structure and the materials used, to increase the features of the electrochromic device.
All of this allows thinner layers to be made and a faster switching speed to be achieved.
The electrochromic material is preferably a material selected from the group formed by transition metal oxides and combinations of at least two of these. Particularly, the transition metal oxides are, for example, wolfram oxide, molybdenum oxide, vanadium oxide, titanium oxide, chromium oxide, iridium oxide or niobium oxide, among the most common transition metal oxides which may have transitions between different states of oxidation (with valence change), associated or not with changes in the coloring of the oxide (from transparent to colored and vice versa, or always transparent).
The layer of electronic insulating transparent ion-conducting solid electrolytic material is preferably a layer of an electronic insulating material having a resistivity above 109 xcexa9xc2x7m (measured as conventional non-nanostructured material) and having a high optical transmittance associated with a low extinction coefficient, k less than 10-2, in the visible range (400 nm to 800 nm). The extinction coefficient is related to the absorption coefficient by the following formula (1):
xcex1=4xcfx80k/xcexxe2x80x83xe2x80x83(1) 
and the transmittance is related to the absorption coefficient by Lambert""s law (2):
T(in units)=I/Io=exe2x88x92xcex1dxe2x80x83xe2x80x83(2) 
These electrolytic materials are advantageously selected from the group formed by oxides, nitrides, oxinitrides and carbides of silicon, fluorides, oxides and nitrides of semi-metals, and combinations of at least two of the foregoing. Particularly, they are metals of the group formed, for example, by the binary compounds SiO2, SiO, SiC, Ta2O5, Al2O3, Si3N4, Y2O3, MgF2, Zr3O2, the ternary compounds LiAlF4, LiNbO5 and combinations of at least two of any of the foregoing compounds. These materials are those which, among the most common transparent electronic insulating (dielectric) materials, have most appropriate ion transport properties for their application to an electrochromic device.
A preferred embodiment of the invention is obtained when the superimposed layer structure comprises a first layer which is an electrode, being a conductive metal or a transparent conductive oxide, a second layer which is an electrochromic material, a third layer which is an electronic insulating transparent ion-conducting solid electrolytic material, a fourth layer which is also an electrochromic material, with the electrochromic material of the fourth layer being complementary to the electrochromic material of the second layer and a fifth layer which is an electrode, being a conductive metal or a transparent conductive oxide. If one of the layers forming an electrode is a reflective layer, or if an initial or final reflective layer is added to the device, this may, for example, be applied to the manufacture of mirrors, for example, rear-view mirrors for vehicles. Alternative, other articles may be manufactured, such as window glasses for vehicles having a variable transmittance either over the whole glass or in an area thereof. Generally speaking, it is possible to control the electromagnetic energy reflected by said device or transmitted through said device.
Each of the nanostructured layers may be a single layer or may, in turn, be formed by a number of nanolayers (sub-layers of nanometric thickness), which may be made from different materials, or of the same material but applied or deposited under different conditions. In this way, the properties of each of the nanolayers may be combined. For example, one advantage of a layer of electrolytic material formed by nanolayers is that the electronic barriers are increased, while the total thickness of the layer may be reduced, whereby the switching speed of the device may be increased, since the distance that the ions have to cover between both electrochromic layers is reduced. A layer of electrochromic material having nanolayers has a more pronounced nanostructure, thereby increasing the number of grain boundaries or the interfaces between crystalline domains inside the nanolayers, whereby the chemical instability of the material is increased by increasing the defects in the boundaries and at the ion exchange sites. On the other hand, the nanolayers allow the level of tensions generated due to: 1) the different thermal expansion of the adjacent layers during the deposition process; 2) the sputtering with atomic oxygen or argon ions during the deposition process (this sputtering is part of the manufacturing process of the nanostructured layer, as will be seen hereinafter); 3) the expansion of the layers associated with the electrochemical process of insertion of ions in the structure, and/or 4) the expansion of the layers caused during the ion exchange process between the electrochromic layers, in an operative cycle of the electrochromic device.
The thickness of said nanolayers is preferably comprised in the range of 2 to 20 nm, and the total thickness of each of the nanostructured layers is preferably comprised in the range of 100 to 400 nm.
An interface layer having a thickness ranging from 0.5 to 10 nm is advantageously inserted between at least two of said layers. This interface layer between the electrochromic layers and the insulating layers or the electrodes may be more or less gradual and may perform a buffer effect between the layers, in such a way that the tension due to thermal expansion, to ion insertion while the device is operating (which causes the interatomic distance to vary), or to the lack of adaptation between the crystalline (or nanocrystalline) network of the different materials in contact is reduced. This allows certain characteristics, such as for example the adherence of the layers to each other and to the substrate, to be improved.
Thus, the layer structure may be embodied with reduced thicknesses. It is, in particular, possible to make electrochromic devices according to the invention in which the thickness of the layer structure is less than 5 xcexcm.
It is possible to add a passivating coating of a material selected from the group formed by SiO2, polymeric siloxane, and mixtures thereof, to the electrochromic device. This allows the layer structure to be protected from the environment and, possibly, one of the glass supports to be dispensed with. This layer is preferably more than 1 xcexcm thick.
The complementary electrochromic material habitually fulfils two functions: on the one hand, it has an ion storage function, and on the other hand, it adds its electrochromic effect to that of the other electrochromic layer. With nanostructured layers, it is possible to improve the efficacy of the electrochromic layer in such a way that a second layer of electrochromic material is no longer necessary. In these cases, it is possible to replace the layer of complementary electrochromic material with a layer of material which, while not having electrochromic properties, may act as an ion store. Examples of these materials are the transition metal oxides having low electrochromism in the visible range (chromium oxide, titanium oxide, manganese oxide, cerium oxide, among others) and which may reduce or increase their oxidation state with the insertion of alkali ions.
The electrochromic device of the invention is formed by a plurality of solid-state material layers. This advantageously allows non-planar electrochromic devices, i.e., in which the substrate is not flat, but has some type of curvature, to be made.